In the field of electronics, major properties required for a polymer material include moldability, heat resistance, durability, high conductivity, corrosion resistance, recyclability and electromagnetic wave blocking effect, depending on a product and an application. In general, polymer materials used in this field include thermosetting resins such as epoxy resins and phenol resins; and engineering plastics such as polyphenylene oxides, liquid crystal polymers, polyimides and polycarbonates.
However, although a material generally having the above properties is strongly needed, producing such a material is generally difficult, often resulting in a disadvantageous cost. One of such required properties is higher conductivity, and in addition to the property, there is needed a polymer material further exhibiting good corrosion resistance.
Japanese Patent Publication No. 1991-77288 has proposed a fine carbon fiber with higher conductivity as a conductive material. However, when being combined with a resin, such a fine carbon fiber is less dispersible, leading to insufficient conductivity. Specifically, as described in Japanese Patent Laid-open Publication No. 1995-102112, a product obtained by blending a resin (80% by weight) with a fine carbon fiber (20% by weight) in a dry blending process and then molding the mixture by an extruder has a volume resistivity as high as about 1 Ω, which is insufficient conductivity. Japanese Patent Laid-open Publication No. 1991-55709 has described a conductive sheet wherein a particular hollow carbon fibril is dispersed in an insulative polymer material, but it has an inadequate volume resistivity.
One of applications requiring conductivity and corrosion resistance is an electric double layer capacitor using an aqueous electrolytic solution. An electric double layer capacitor is an electric element consisting of a pair of polarizable electrodes, collectors and an electrolyte. Such capacitors can be generally classified into those comprising an organic solvent type electrolytic solution or an aqueous electrolytic solution.
Comparing these two types of capacitors, a capacitor comprising an aqueous electrolytic solution can generally give a lower output voltage than a capacitor comprising an organic solvent electrolytic solution, but it has larger ion conductivity in the electrolytic solution, so that an internal resistance of the capacitor may be reduced to give an advantageously larger output current. Furthermore, a capacitor comprising an aqueous electrolytic solution is advantageous in that handling thereof is easy from a view point of safety, because it does not comprise a flammable liquid as used in a capacitor comprising an organic solvent electrolytic solution,
An electric double layer capacitor comprising an aqueous electrolytic solution has a configuration as shown in FIG. 1, where polarizable electrodes 2 are placed opposite each other via a separator 4, collectors 1 are placed outside of the individual polarizable electrodes 2 and are insulated by a gasket 3.
When using a plurality of capacitors are connected in series or in parallel for obtaining a higher output voltage in an electric double layer capacitor comprising an aqueous electrolytic solution, an internal resistance of the capacitor-composite as a whole may be increased, resulting in a lower output current. Thus, it has been desired to reduce an internal resistance of each capacitor as much as possible.
It is well-known that an internal resistance in each capacitor is generated from an aqueous electrolytic solution, polarizable electrodes, collectors and interfaces therebetween. Reduction of internal resistance in a capacitor has been conducted by, for example, reducing a volume resistance in the collector.
Commonly used collectors include conductive rubber films (for example, see Japanese Patent Laid-open Publication Nos. 2000-12388 and 1993-94925). These comprise a rubber containing conductive carbon and the like and have generally a volume resistance of about 10 to 100 Ωcm. Thus, there have been needs for a material having a further lower volume resistance.
Additionally, an electric double layer capacitor comprising an aqueous electrolytic solution comprises an about 25 to 50% aqueous sulfuric acid solution as an electrolytic solution. A collector must be, therefore, also acid resistant.
A known example of the above collector is a conductive resin film in which a conductive material is a metal (see Japanese Patent Laid-open Publication No. 2000-12388), but it has a drawback that its conductivity is unstable under an acidic environment. When using a noble metal with good corrosion resistance as a conductive material, a cost becomes extremely higher, while a carbon-containing conductive material has lower conductivity than a metal, resulting in inadequate conductivity.
List of References
For a conductive resin film, Japanese Patent Publication No. 1991-77288 and Japanese Patent Laid-open Publication Nos. 1995-102112 and 1991-55709;
For an electric double layer capacitor, Japanese Patent Laid-open Publication Nos. 2000-12388 and 1993-94925.